A fuel cell stack is comprised of a plurality of individual fuel cells which are stacked in electrical series relationship. A separator plate separates each fuel cell. An electrode such as an anode is adjacent separator plate followed by a matrix which includes electrolyte. Beyond the matrix is another electrode in the form of a cathode followed by another separator plate. Arrangements are made to pass reactants in the form of fuel and oxidant through open spaces provided between the separator plates and a corresponding electrode.
The electrolyte diffuses through the electrode so that a reaction takes place between the fuel and the electrolyte on the surface of one electrode while another reaction takes place between the oxidant and the electrolyte at the surface of the other electrode. These reactions and the details of the electrodes and the electrolyte as well as the matrix holding the electrolyte are well discussed in the prior art.
Material selection for each of the components is limited by the corrosion resistance of the material to the particular substance in the fuel cell as well as the temperature level at which the fuel cell is operating. Each component used must therefore be selected of a material which will resist corrosion.
In accordance with the fuel cell chemical reaction, a flow of electrons must occur between adjacent fuel cells. Accordingly, the components must provide good electrical conductivity to facilitate the flow without heating loss and must also be in good electrical contact to pass the current.
An axial compressive load must be maintained on the stack for the purpose of maintaining appropriate contact throughout the cell throughout temperature transients and differential expansion in different areas.
In molten carbonate cells, the cathode tends to be frangible and rubble like. The more common means of providing a gas flow path between the separator and electrode cannot be used. Electrodes manufactured with grooves extending therethrough to provide this gas space have insufficient strength to survive.
It is also noted that the reactant must not only obtain access to the gas space, but must pass through the gas space in a continuous flow. In order to avoid inappropriately high pressure drops and velocities, adequate flow area must be achieved. This must be obtained in such a way that the gas has reasonable access to the electrode itself without significant blanking of the electrode by any apparatus which may be in contact with it.
In prior art molten carbonate cells the cathode has presented a particular problem. A current collector for passing the electrons from the cathode to the separator plate must be supplied, and supplied in a manner to permit gas access to the cathode and sufficient flow space to achieve an appropriately low pressure drop.
In the prior art, a perforated plate having a substantially flat surface is located in contact with the cathode to provide support and good electrical contact. A folded kintex piece which is generally in the form of a metal corrugation with openings therethrough is located between the perforated plate and the separator. This provides passage for the reactant gas such an arrangement requires multiple pieces usually with a material requirement which is 2.6 times the area of the cathode. It also requires additional vertical space since in addition to the flow area blocked by the kintex, the thickness of the perforated plate exists which is not available for transverse flow. It furthermore has an additional electrical contact joint between the perforated plate and the kintex.
As described before, the fuel cell is under compression at a loading of at least 170 kilopascals. The current collector should have a sufficient flexibility to maintain contact over temperature transients and flexibility to accept manufacturing tolerances. It also must adequately accept this load without excessive creep.